In order to discover and determine the topology of a communication network, it may be required that network devices report knowledge of their local topology to a network management device. Several products exist on the market, such as, for example, Hirschmann's HiVision, wherein protocols like ARP (ARP: Address Resolution Protocol), ICMP (ICMP: Internet Control Message Protocol), or SNMP (SNMP: Simple Network Management Protocol) are used. These tools operate on layer 3, for example, using IP addresses (IP: Internet Protocol) of the network devices, and are not directly aware of layer 2 devices or configurations, such as, for example, media converters, repeaters, unmanaged bridges or switches operating on layer 2 only.
On the link layer, namely on layer 2, the topology of communication networks may be discovered using the vendor-neutral Link Layer Discovery Protocol (LLDP, IEEE 802.1AB) or using vendor-specific protocols such as Microsoft's Link Layer Topology Discovery (LLTD), the Cisco Discovery Protocol, or any other vendor-specific protocol. In the LLDP, network devices send through each of their network interfaces, at a fixed interval, a so-called Link Layer Discovery Protocol Data Unit (LLDPDU) in the form of an Ethernet frame, which has its destination MAC address (MAC: Media Access Control) set to a specific multicast address. Information gathered with LLDP is stored in the network devices in a management information database (MIB) and may include system names, port names, VLAN names, etc. The MIB of the network devices may be queried with the SNMP in order to discover the network nodes and establish the topology of a network in which all devices are LLDP-enabled. The latter prerequisite, however, is not fulfilled in most automation networks deployed today.